Two Sum

Python data engineering interview problem. Difficulty: beginner. Pattern: Hashing. About 10 minutes. Free to practice.

Return indices of two numbers that add to target. Treat this as a production helper: match the contracted return shape, including empty and duplicate inputs.

A classic question: given a list of numbers and a target, find two different numbers in the list that add up to the target, and return their positions. Write a function two_sum(nums, target) that returns a list [i, j] with i < j, so that nums[i] + nums[j] == target. Positions start at 0. You can assume there is exactly one valid pair, and you may not use the same position twice. For [2, 7, 11, 15] and a target of 9, the numbers 2 and 7 add up to 9. They sit at positions 0 and 1, so the answer is [0, 1]. Keep the print lines in the editor.

Requirements

• Return the 2 indexes, not the 2 values.
• The smaller index comes first, so i < j.
• The two numbers at those indexes must add up to target.
• Do not use the same index twice.

Constraints

• There is exactly one valid pair, so you do not need to handle 'no answer'.
• Indexes start at 0.
• A value may appear more than once in the list, such as [3, 3] with target 6.

Examples

Input: two_sum([2, 7, 11, 15], 9) Output: [0, 1] 2 + 7 = 9.

Topics: lakebench, python, indices, hash map.

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